U.S. Pat. No. 4,232,961 shows multicolor imaging apparatus in which three or four imaging members are separately charged, exposed and toned to create toner images in different colors and those toner images are transferred in registration to an intermediate web from which they are transferred to a receiving sheet. Apparatus with separate imaging members for creation of separate color toner images is many times faster than present commercial devices in which the images are formed consecutively on a web or drum photoconductor.
A number of other references show separate imaging members which create separate color toner images and transfer them directly to a receiving sheet. See, for example, U.S. Pat. No. 4,464,501 and U.S. Pat. No. 4,162,843. In U.S. Pat. No. 4,580,889 the transfer sheets are presented to the imaging members by attaching them to the surface of a drum rather than a moving web which is shown in the rest of the above references.
Maintenance of cross track and skew registration in combining separate toner images from different sources is an extremely challenging task. In devices in which the images are transferred to a web (U.S. Pat. No. 4,232,961) or transferred to a receiving sheet carried by a web (U.S. Pat. No. 4,464,501 and U.S. Pat. No. 4,162,843) any variation in web tracking between transfers will cause misregistration.
The approach shown in U.S. Pat. No. 4,580,889 (see also Japanese Kokai 63/96675 published Apr. 27, 1988) in which the members carrying the toner images are positioned around a drum and the receiving sheet is tacked to the drum essentially solves this cross track registration problem. That is, the receiving sheet, once tacked to the drum, will follow the drum through all four transfers. The drum itself can be controlled to avoid cross track registration problems.
In order to provide the space on the periphery of the drum for four separate imaging members, as well as the toner supply for each member, the drum itself must be of substantial diameter. This is especially true if the developing mechanism for creating the toner images works best in limited orientations, for example, if it prefers to tone an image generally above the toning station.